Like so many American teenage romances, it all began in a shopping mall - a 14-year-old schoolgirl smitten by an intriguing boy three years her senior. They exchanged text messages, held hands in the back row of the cinema and, when the girl's family heard that he was an orphan, they invited him to move in with them.

But the girl discovered a shocking secret about her 17-year-old boyfriend, Mark Villanueva, after the pair were pulled over by a traffic policeman near their home in the Seattle suburbs.

"Mark" turned out to be a 30-year-old woman called Lorelei Corpuz who had fooled everyone about her true identity for more than a year.

"They were all surprised that this individual was not who she said she was, both in name and sex," said Sergeant Robert Goetz of the Everett police department in Washington state.

Last night the girl and her family were trying to figure out how they had fallen for the con, while Ms Corpuz remained in the Snohomish county jail, charged with child rape and molestation.

"It was a regular teenage girlfriend-boyfriend thing: like holding hands, hugging, kissing," the girl said in an interview with the Seattle Times. "He was really nice, and I didn't really have anybody to talk to because my mom and dad are always at work. He talked to me."

Later, she said, her "boyfriend" became jealous of the time she spent with friends of her own age, and began to beat and bite her. "She made it like a serious relationship - like we're married," the girl added.

The teenager told police she had met "Mark" in the food court of Everett's Alderwood Mall in September 2005. They exchanged mobile phone numbers and began calling each other before going on a date two days later.

Court papers say Ms Corpuz falsely told the victim that her mother had died of cancer and father was killed in a car accident, prompting the girl's family to invite her to live with them and share a room with their son. The girl said "Mark" would sneak into her bedroom to have sex when her parents were out.

"Over the year that suspect lived with victim, the suspect never let victim see her/his private parts," an Everett police officer, Don de Nevens, wrote in his court submission. "Victim always thought that suspect was male until officer informed her otherwise."

Ms Corpuz's ruse was discovered when police stopped the car she and the girl were in at a petrol station. An officer checked the licence plate and found an outstanding traffic warrant in the name of Mark Villanueva from the neighbouring town of Marysville, but recognised Ms Corpuz as a woman from an earlier arrest.

The officer asked the girl why she was in the car with a woman. "She gave us a curious look when the officer said 'woman'," said Sgt Goetz. "She said, 'That's my boyfriend.' That obviously piqued the concern of the officer."

In January a convicted sex offender, Neil Havens Rodreick, 29, was arrested for posing as a 12-year-old boy and trying to enrol at a school in Arizona. But criminologists say it is rare for an adult to pose successfully as a younger member of the opposite sex.